Communication signals typically comprise analog or digital signals modulated onto a carrier signal, such as a radio frequency ("RF") carrier signal. The information to be communicated is generated as a baseband signal, which is then modulated onto a carrier signal which is transmitted over a transmission medium, such as the atmosphere or a wired communication network such as a cable television ("CATV") network. To increase the capacity of communication networks, the use of digital signals modulated onto RF carrier signals, or digitally modulated signals, has been on the rise.
One communication industry that is increasingly using digitally modulated signals is the CATV industry. CATV systems are communication systems that commonly include an antenna and receiver to receive a RF television signal, a cable transmitter linked to the receiver for the purpose of re-broadcasting the received signal, and a cable distribution network. The cable distribution network commonly includes a coaxial wire cable that extends from the cable transmitter to a large number of subscribers. This cable distribution network can be quite large, and can include many miles of cable, as well as repeaters, splitters, and other network components.
Historically, CATV systems have employed analog modulated signals to rebroadcast television signals. Such analog modulated signals included, for example, as frequency modulated, amplitude modulated, and phase modulated signals. Recently, however, CATV systems have employed digitally modulated RF signals to transmit television signals. Digitally modulated signals are preferred because they can compress the information content of several television signals into the bandwidth normally allocated for a single analog modulated television signal.
At present, CATV systems employ digital modulation techniques that include quadrature amplitude modulation (QAM) and carrierless amplitude/phase modulation (CAP) techniques, to transmit digital signals over a cable distribution network. For example, a QAM16 signal is a 16-state quadrature amplitude modulation that employs four phasors separated in phase by 90 degrees, with each phasor amplitude modulated to one of four levels. This provides sixteen phasors with each one representing a unique 4-bit binary code group. When a QAM16 signal is used, the transmitted signal strength drops off sharply at the upper and lower frequency limits of a given channel, and for a given transmitted bit rate, the bandwidth is smaller than in some other modulation methods. The digital QAM16 channel, therefore, presents a very small and acceptable amount of interference to any adjacent channels. QPR and CAP techniques operate on different principles, but have the same general result. QAM, QPR and CAP digital modulation techniques are well known to those of ordinary skill in the art.
Of the above general modulation techniques, CATV systems presently use QAM64, QAM256, QPR, and CAP-16 to effectuate modulation of digital baseband signals in a highly compressed format. These digital modulation techniques produce a summation of pseudo random sinusoids that are similar in randomness to white noise.
The advent of the use of digitally modulated RF signals in CATV systems has necessitated the development of methods and devices for measuring such signals. In particular, CATV service providers often perform signal strength measurements to determined the condition of the network and the quality of signal transmission thereon. Accordingly, a reliable method of measuring signal strength of digitally modulated signals is necessary.
The signal level measurement of a CATV system has many uses, including detecting and diagnosing faulty equipment, detecting faults in the cable distribution network, and achieving optimum performance in the CATV system. The complexity and size of a cable distribution network require that network operation and performance be periodically tested and monitored.
The signal level measurement technique historically used for analog modulated television signals consisted of a measurement of the amplitude of standard pulse signals, such as the horizontal or vertical synchronization pulses, within the baseband television signal. Such a measurement technique required demodulation of the television signal in order to facilitate isolation and measurement of the pulse signal therein.
Digitally-modulated signals, however, because of their pseudo-random nature, may be measured using simple root mean square ("RMS") power measurement techniques. In particular, RMS power measurement techniques, which measure the spectral energy of the signal under test, cannot be used for analog modulated television signals because signal energy in such signals is greatly dependent on video content. In other words, two signals that are transmitted with the same power may measure different RMS power levels based on whether the program video content has a lot of white information or a lot of black information. Accordingly, a simple RMS power measurement of an analog CATV signal does not provide a reliable indicator of the quality of signal transmission through the CATV network. Digitally-modulated signals, however, are pseudo-random in nature, and thus there is little or no correlation between the program content and the measured RMS signal power. As a result, RMS power measurements are presently employed to measure the signal level of digitally modulated RF signals in CATV systems.
RMS measurements may be carried out by analog or digital methods. For increased accuracy as a function of component cost, digital RMS methods have increasingly been used. In addition, digital circuitry, which typically includes a programmable processor of some type, provides additional flexibility which allows the measurement device to carry out additional types of CATV signal measurements with little or no additional circuitry.
One drawback to the use of digital measurement techniques arises in the context of the conversion of the analog signal to a suitable digital representation of the analog signal. In particular, in order to carry out a digital RMS measurement, the input signal must first be converted to a digital signal. That conversion is typically carried out by an analog to digital ("A/D") converter that samples an analog signal to produce a digital signal therefrom.
A fundamental requirement of analog to digital conversion is that a sampling A/D converter must typically have a sampling rate that exceeds twice the highest frequency component of the signal being measured. In other words, if an analog signal to be converted has a highest frequency component of 1 MHz, then the A/D converter used to convert that signal to a digital signal must have a sampling rate that exceeds 2 MHz. The sampling rate requirement is known as the Nyquist criterion.
The drawback presented by the A/D conversion is that the generation and subsequent processing of digital samples having a sample rate that satisfies the Nyquist criterion can significantly increase component cost of the measurement device. For example, there is a limitation to the sampling rate that is achievable in commercially available, low cost, A/D converters. More importantly even if sufficiently high sampling rate A/D converters are available, there is a cost limitation to the ability of subsequent digital signal processing equipment to process samples generated at such a high sampling rate. The relatively high frequencies of CATV signals further aggravate these cost issues. For example, a CATV television signal may have a carrier frequency of 100 MHz and a bandwidth that extends to 104 MHz. As a result, an A/D converter having at least a 208 MHz sampling rate would be required to satisfy the Nyquist criterion. Moreover, a digital circuit used to perform an RMS power measurement of the signal must employ digital components that are capable of processing samples at that rate. Such devices are either presently unavailable or cost prohibitive.
To reduce the processing speed requirements of such devices, CATV measurement devices using digital measurement techniques typically perform a frequency conversion on the received signal to produce a signal having a relatively low intermediate carrier frequency ("intermediate"). For example, such a device could convert the signal with the 100 MHz carrier such that it had a 100 kHz intermediate carrier. In such a case, the highest frequency component would be approximately 4.1 MHz (i.e., 100 kHz+4 MHz) instead of 104 MHz. Accordingly, the digital devices need only have a processing speed that corresponds to the Nyquist criterion with respect to 4.1 MHz.
While down conversion reduces the processing speed requirements of the digital measurement circuits to some degree, the processing speed required even for such down-converted CATV signals remains relatively high, and contributes significantly to the cost of the circuit. There is a need, therefore, for a device that measures power of digitally modulated communication signals without requiring high speed digital processing circuitry.